In recent years, a metal oxide having semiconductor characteristics, which is called an oxide semiconductor, has attracted attention as a novel semiconductor material for a semiconductor element having high mobility provided by polysilicon or microcrystalline silicon and having uniform element characteristics provided by amorphous silicon. The metal oxide is used for various applications; for example, indium oxide is a well-known metal oxide and used as a material of a pixel electrode in a liquid crystal display device. Examples of such a metal oxide having semiconductor characteristics include tungsten oxide, tin oxide, indium oxide, and zinc oxide. Transistors in which a channel formation region is formed using such a metal oxide having semiconductor characteristics have been already known (Patent Documents 1 and 2).